1. Field of the Invention
The present invention relates to viscosity-stabilized aqueous polyester dispersions, to processes for preparing them and to their use.
2. Description of Related Art
The use of silica, or silicon dioxide, and/or hydrolyzable silicon compounds for improving certain properties in coating compositions has long been known. For instance, U.S. Pat. No. 2,432,484 describes the use of colloidal silicon dioxide in the form of a sol in water or other appropriate solvents, such as glycol ethers, for improving the abrasion resistance of the coatings it is used to produce. EP-A 0 180 129 discloses the use of amorphous, porous silica with a specific surface area of from 300 to 900 m.sup.2 /g in combination with alkylalkoxysilanes as a coating on plastics to improve their scratch resistance.
U.S. Pat. No. 3,986,997 describes aqueous coating compositions based on silanol condensates and on silicon dioxide, consisting of a dispersion of colloidal SiO.sub.2 in a solution of a partial condensate of an alkylsilanol. Similar coating compositions are known for example, from DE-A 2 811 072, and contain from 30 to 50% by mass of colloidal silica and from 50 to 70% of siloxane. A coating composition comprising partially hydrolyzed alkylalkoxysilanes in alkaline solution, and silicon dioxide, is described in U.S. Pat. No. 3,324,055. U.S. Pat. No. 4,526,910 describes the preparation of colloidal silica in organic solvents by controlled hydrolysis of tetraalkoxysilanes in these solvents.
DE-A 30 14 411 describes an aqueous coating composition consisting of a melamine-formaldehyde resin, colloidal silicon dioxide, and an alkylalkoxysilane of the form RSi(OR).sub.3. The use of SiO.sub.2 and silicon compounds is restricted essentially to improving the mechanical properties of coatings produced using the coating compositions thus modified.
There is as yet still little literature on improving Theological properties of coating compositions. EP-A 0 287 589 is directed to the use of alkylalkoxysilanes and aminoalkyltri-alkoxysilanes for functionalizing heteropolysaccharides, as thickeners for thixotropic coating compositions. EP-A 0 608 107 describes the preparation of externally emulsified, aqueous silica dispersions and aluminum silicate dispersions, which are formed by emulsifying pyrogenic (fumed) silica or aluminum silicates by means of auxiliary dispersants (emulsifiers) which form polyanions (metaphosphoric acid, polyphosphoric acid and the salts thereof and also boric acid and its salts are mentioned by way of example). The particle size of the resultant silica dispersions lies between 60 and 500 nm and depends very heavily on the concentration of emulsifier. The silica dispersion that is stabilized acidically in this way (that is, in an acidic environment) is used to increase the level of corrosion protection of aqueous coating compositions, in particular, epoxy resin dispersions.
However, alkali-stabilized aqueous silica dispersions (that is, non-externally emulsified, acidic silica dispersions, as described above) and alkoxysilanes or their successor products have not yet been mentioned in connection with polyester dispersions.
Polyester resins can be prepared in a known manner by polycondensation, preferably in the melt, of one or more polyfunctional organic hydroxy compounds with one or more polyfunctional organic carboxylic acids, with or without the addition of monofunctional aliphatic saturated or, preferably, unsaturated carboxylic acids. The terms "polyester resin" and "alkyd resin" are used here synonymously.
Aqueous dispersions of such polyester resins are usually stabilized anionically. This means that in the course of polyester synthesis, the polycondensation is terminated at a certain residual acid number (corresponding to a residual content of free carboxyl groups which is other than zero) and these acid groups (some or all of them) are then neutralized with bases, preferably with ammonia or amines, but also with alkali metal hydroxides or alkaline earth metal hydroxides. The neutralization forms strongly polar salt groups, and the polyester can then be dispersed in water.
The polyester resin dispersions obtained in this way are alkaline, generally having a pH of more than 7, preferably in the pH range from 8 to 10. In an aqueous medium they undergo (alkali-catalyzed) hydrolysis of the ester bonds, which may lead, inter alia, to a sharp reduction in viscosity over the course of a few months. Systems of this kind therefore not only have short storage and processing lives, but their technical usefulness may be significantly restricted as a result. This lack of stability is observed not only with the polyester resins of the composition described above (referred to below as "unmodified"); it also affects polyester resins modified with drying or nondrying fatty acids, and also acrylic-, urethane- and epoxy-modified polyester resins.
It has now been found that aqueous dispersions of modified or unmodified polyester resins which additionally comprise an alkali-stabilized aqueous silica dispersion are generally stable on storage and show substantially no drop in viscosity on storage at room temperature (from 20 to 25.degree. C.) over a period of at least 6 months.